Geoscience Reference
In-Depth Information
are also avoided in order to avoid plume influ-
ence. We can ask the question 'Can we disprove
the null hypothesis that OIB and MORB samples
are drawn from the same population distribu-
tion function?' The answer is no, meaning that
the datasets could be drawn from the same pop-
ulation. Previous studies have reached the oppo-
site conclusion by comparing extreme values in
one population with the mean value, or range,
of the other population after the extreme values
are removed. The inter-element isotopic ratios of
MORB and OIB, however, are from distinct popu-
lations. This can be attributed to degassing and
contamination processes rather than to source
characteristics.
Table 16.2 compares the results for MORB (fil-
tered and unfiltered datsets), BABB, OIB and near-
ridge seamounts. The dates of the compilations
of MORB are given. The means have changed little
with time, in spite of the large expansion of the
data. This is due to the exclusion of large ratios
as being due to plume influence.
OIB samples show great diversity and exhibit
values both higher and lower than found along
most ridges but the median, a robust measure
of central tendency, of the two populations is
the same, about 8.5---9.0
R
A
. It is true that more
extreme values are found in some OIB samples,
but this is predicted by the CLT if spreading
ridges sample a larger volume of the mantle than
do oceanic islands. The means of various MORB
and OIB datasets are usually within one standard
deviation of each other. [
statistics of man-
tle helium
]
The central limit theorem is illustrated in
Figure 16.7.
